Long mission tethered aerostat and method of accomplishing

ABSTRACT

Apparatus and method for a continuous replenishing of the lift gas and maintaining the proper pressure and lift of a tethered medium altitude aerostat utilizing a novel feed tube running the entire length of the tether. A first end of the feed tube is connected to the aerostat while a second end is connected through a novel slip ring means to a pressure controlled helium ballast chamber and scrubber on the ground to maintain pressure and lift by a reversible compressing pump. A plurality of pressure and temperature sensors and tension gauges strategically placed inside and around the airship continuously monitor the temperature and pressure changes in the aerostat. A data retrieval and communication unit mounted on the aerostat collects measurement data from the pressure, temperature and tension sensors which is relayed to the ground station and the data is used to regulate lifting gas pressure inside the aerostat.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 11/790,899 filed Apr. 27, 2007 now U.S. Pat. No.7,708,222 issued May 4, 2010.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON COMPACT DISC

Not applicable.

REFERENCE TO A “MICROFICHE APPENDIX”

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains generally to the field oflighter-than-air airships. More particularly, the invention relates tothe control and regulation of a lift gas such as helium along with itspressure within a tethered helium filled aerostat using a ground basedlift gas ballast chamber. The invention includes a novel hollow tetherwith a lifting gas feed tube and novel double slip rings which allowweathervaning by providing a substantially airtight connection betweenthe ground and the aerostat.

2. General Nature of the Prior Art

A tethered aerostat is an aerodynamic shaped, lighter-than-air vessel ofa flexible structure filled with a lifting gas such as pressurizedhelium and mechanically anchored with a long high strength tether to aground structure. An aerostat is equipped with a system of sensors,blowers and valves, which, in conjunction with a plurality of deformableair compartments called ballonets are used to control the pressurewithin the hull to maintain aerodynamic shape to minimize the drag forceexerted on the airship by the ever present wind. To compensate for thediurnal and seasonal variation of ambient temperature as well as thesolar gain during the daylight period, the volumes of air inside theballonets are changed either by opening valves to allow air to be pushedout of the ballonets or by turning a blower on to blow outside air in topressurize the ballonets to maintain pressure within the airship. Theballonets are also used to alter the aft-fore balance of the lift forcein order to provide pitch control. The power needed on board theaerostat is delivered through the high voltage power cables embeddedwithin the tether. The tether also contains one or more optical fibersto enable onboard equipment to communicate with the ground station.

3. Description of Related Art Including Information Disclosed Under 37CFR 1.97 and 1.98

In the prior art due to inherent helium leakage and lift gascontamination due to an air infiltration, a tethered aerostat needs tobe brought down periodically to refill the helium gas and to performregular maintenance. A typical tethered blimp with a helium volume ofbetween 50,000 cubic feet and 100,000 cubic feet can be expected to stayup for only about 10 to 20 days. The aerostat is retrieved from altitudeby drawing the tether back through a mechanized winch. As the aerostatis lowered, the helium gas contracts owing to the increased ambientpressure at lower altitude. This is compensated by pumping air into theballonets to maintain the aerodynamic shape. To launch or re-launch theaerostat, the tether is gradually released to allow the aerostat toascend to altitude, the expanding helium gas forces air from theballonets through the air valves. The valves are controlled to preventthe helium pressure from dropping below a threshold level.

In addition to helium leakage, there is also the issue of airinfiltration which occurs at a slower rate. Since air can not providelift, the air inside the helium bag needs to be purged periodically.This is accomplished when the aerostat is returned to earth formaintenance with a helium scrubber which removes the air from thehelium. Although it is possible to include helium scrubbing equipmentaloft such as described in Haunschild U.S. Pat. No. 5,090,637. Howeverto remove the air while the airship is aloft for the purpose ofextending the mission duration is highly unattractive due to theadditional weight, power requirements, and complexity, hence heliumscrubbing is usually done on the ground.

Haunschild U.S. Pat. No. 5,090,637 recognizes the importance of removalof oxygen, nitrogen and other gases from helium. Haunschild however doesmaintenance on the lift gas either in the airship or on the ground. Theknown prior art does not provide for the maintenance of the lift gas onthe ground while the airship is deployed by providing the bidirectionaltransfer of the lift gas between the ground and the aerostat.

The relatively short mission duration and the significant downtimeinvolved in retrieving and re-launching the aerostat, as well as thetime it takes to replenish and clean the helium makes it unappealing touse a tethered aerostat for tasks that demand high availability such asweather monitoring and telecommunications. Frequent launch and retrievalalso drastically increase the risk of damaging the envelope of theaerostat, thereby shortening the service life of the entire system.

The use of ballonet to maintain the excess pressure calledsuper-pressure to combat the diurnal pressure fluctuation of the heliumgas also produces some unwelcome side effects, the primary of which isthe cyclic variation of the lift force resulting from the cyclicexpansion and contraction of the helium volume. Since the aerostat isphysically constrained by the tether, this leads to the cyclic variationof the tension on the tether embedded cables and optical communicationfibers. Such cyclic variation progressively weakens the tether overtime. In addition, the constant switching of the blower on or off alsodrastically shortens the service life of the blower. Malfunction of theblower can cause the aerostat to lose its pressurization resulting indamage to the aerostat by the wind aloft by a sudden increase in winddrag. Consequently, the blower needs to be maintained and/or replacedwith high frequency that can be reduced by maintaining the pressure ofthe lift gas by a bidirectional transfer of lift gas between the airshipand the ground. Furthermore, ballonets and the attendant blowerequipment of the prior art increases the size, weight, and cost of theairship.

Some very low altitude tethered aerostat systems deployed at an altitudeof up to 300 feet use a feed-tube that either is embedded within thetether or runs parallel to the tether to allow the refilling of thehelium to be performed while the aerostat is aloft. This is accomplishedby connecting the proximal end of the feed-tube to a bottle ofcompressed helium gas and opening the valve partially to send a burst ofhelium gas up the feed-tube to replenish the helium. This prior art doesnot provide a bidirectional flow of lift gas or do maintenance on thelift gas by scrubbing the lift gas. In addition such known prior artfeed tubes which would have wall thickness sufficient to accommodatehigh pressure bursts of helium could not be used in airships deployed at5,000 feet due to weight which would limit the altitude of the airship.Further the known prior art has not provided for a transfer of lift gasin a weathervaning airship through airtight slip rings.

In addition in the prior art the relatively high helium pressure used invery low altitude tethered aerostat forces the helium gas up with a flowvelocity well in excess of 50 m/s for altitude of up to 300 ft. Such ahigh flow rate can rapidly heat up the feed-tube and could eventuallydamage the feed-tube if the flow rate is sustained for a long durationand damage the airtight connection of the slip ring to accommodateweathervaning.

In the prior art the feed tube has to be thick walled in order towithstand the pressure employed to force helium lift gas into thepressurized aerostat which makes the tether too heavy for higheraltitude applications. Even at such a high flow rate, the refilling ofthe helium would still take a long time. At higher altitudes, theincreased length of the feed-tube increases the flow resistance, whichdrastically reduces the flow speed, making manual control of therefilling process infeasible. The flow rate can be increased by openingthe valve fully to increase the pressure head. However, this requiresthe wall of the feed-tube to be even further strengthened to enable itto resist the much higher helium pressure head. This dramaticallyincreases the thickness of the feed-tube and since the total weight ofthe feed-tube is proportional to the thickness as well as the length ofthe tube, the concomitant increase in the weight of the feed-tube makessuch a scheme impractical.

The prior art includes a variety of slip rings and flying sheaves toallow weathervaning of the airship during launch, flying or docking.Phipps III, et al. U.S. Pat. No. 4,402,479 provides slip rings forelectrical cables and a flying sheave to accommodate weathervaning.Czarnecki, et al. U.S. Pat. No. 4,675,030 includes electrical equipmentin the tether. Some prior art like Lavin U.S. Pat. No. 6,325,330 haveconductors in the tether while other prior art like Beach et al. U.S.Pat. No. 4,842,221 has a central electrical core surrounded by astrength member. The only patent uncovered with what may be argued as ahollow tether is Schneider U.S. Pat. No. 4,092,827 which provides a ductand tether for transferring collected rain water collected from abovethe sea and transported to a nearby island. Schneider U.S. Pat. No.4,092,827 does not replenish lift gas and does not include cables. Noneof the known prior art has a tether with a hollow core surrounded by astrength member having embedded therein electrical cables and opticaltelecommunications cables that have at each end novel slip rings toenable lift gas to be purified and replenished from the ground.

It would therefore be advantageous to provide a tethered aerostat systemthat can continuously cycle and replenish the helium gas without theairship being brought down. It would be advantageous to regulate thequality of the helium and its pressure from the ground without having tovent or dispose of lift gas when the airship is deployed at altitude.

SUMMARY OF THE INVENTION

The terms airship, aerostat and platform have been used and while theterms airship and dirigible contemplates a self propelled aircraft, theterms balloon and aerostat contemplate and aircraft lacking propulsion.The invention is applicable to blimps, balloons, dirigibles, airships aswell as aerostats which are tethered. As a result hereafter the term“aerostat” or “airship” as used herein and in the claims includes allforms of tethered aircraft whether they include some self propelledmechanism or not.

The invention provides a high altitude tethered aerostat that is capableof replenishing and regulating its internal helium pressure continuouslyfrom the ground through a feed-tube that connects the helium or lift gasenvelope of the aerostat to a ground reservoir without the need orsignificantly reducing reliance on ballonets in the airship therebyreducing the demands on blowers and pumping equipment, and methods foroperating the same.

The feed-tube of the invention is sized to support a bidirectionalcontinuous stream of helium gas at a relatively slow flow speed withoutexcessive pumping pressure. The diameter of the feed-tube is constructedto provide just enough helium flow rate to compensate for diurnalvariations and helium loss through leakage. The diameter of the feedtube is thus engineered to match the volume of lift gas in the aerostat.In an alternative embodiment the diameter of the feed tube may be chosento provide high enough helium flow rate that is fast enough to overcomethe diurnal and seasonal fluctuation of helium pressure.

A first end of the feed tube is connected in a substantially airtightfashion to the airship while a second end is connected through a dualslip ring means to a pressure controlled helium ballast chamber on theground wherein the pressure is maintained by a reversible compressingmeans. A plurality of pressure, temperature sensors and tension gaugesstrategically placed inside and around the airship continuously monitorsthe temperature and pressure changes throughout the airship. A dataretrieval and communication unit mounted on the airship or to a gondolacollects measurement data from the pressure and temperature sensors andrelay the information to the ground station, wherein the data isprocessed and used as a feedback signal for the control and regulationof the helium pressure inside the airship. A constantly running heliumscrubber on the ground is utilized to remove the infiltrated air in theground based helium ballast chamber.

In order to supply refurbished lift gas in a bidirectional feed line andreplenish lift gas as needed a novel slip ring and tether wereconstructed. The novel tether includes a hollow center providing for thebidirectional flow of lift gas at a low flow rate and a low flowvelocity to save weight on a tether that may be 4,000 or more feet inlength. The novel tether in addition includes a strength membersurrounding the hollow center and includes co-axial electrical cablesand communication lines which preferably are telecom optical fibersembedded in the strength member. It will be recognized by those skilledin the art that the tether withstands a number of forces mainly verticalbut also some angular forces due to the catenary form assumed by thetether in a no wind condition. The tether is also subject to rotationalforces due to the weathervaning of the aerostat during launch,deployment and recovery.

These compound forces induced upon the tether must be borne primarily bythe strength member and the novel slip rings utilized to absorb therotational and some of the angular forces imposed during launch,deployment and recovery. The novel slip ring member of the inventionincludes a first slip ring which is substantially free to rotate while asecond slip ring is held in place by an electromagnetic field generatedby an electromagnetic coil. The first slip is free to rotate a number of360 degree turns without rotating the second slip ring until a presetnumber of turns are exceeded at which time the second slip ring isreleased from the electromagnetic force generated by the electromagneticcoil and at which time the second slip ring rotates to remove tension ona coiled feed tube connecting the first slip ring to the second slipring in a fluid tight environment or housing.

The novel slip ring member is preferably connected at both ends of thefeed tube in which one end of the feed tube is connected to the airshipand the other end is connected to the ground based lift gasreplenishment and maintenance facility. Preferably the first slip ringalso includes means for also rotatably engaging the electrical cablesand the optical fiber cables included in the novel tether. Alternativelythe first slip ring can include the rotational engagement of theelectrical cables and the first and second slip rings and can includecoiled optical cable disposed coaxially with the coiled feed cable tomaintain a rotatable telecommunications link.

The novel tether novel slip rings and ground based lift gasreplenishment, maintenance and purification system allows the novelairship to be deployed for longer durations than the normal around 20day period of deployment of the prior art aerostats. The novel aerostatof the invention includes a bidirectional valving system that allows theremoval of lift gas during daylight periods of heating supplemented byoptional ballonets that are filled to maintain the rigidity of theairship and that allow the replenishment of refurbished lift gas atnight and the deflation of the optional ballonets at night orcombinations thereof as a result of temperature and pressure variations.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of the invention will become more apparent fromthe detailed description of the invention and disclosure of the bestmode in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic elevational view of an exemplary helium regulatorysystem with feed-tube and ground based helium ballast tank for atethered aerostat;

FIG. 2A is a sectional view taken along the line 2-2 of FIG. 1 of thenovel tether with an embedded bidirectional helium feed-tube;

FIG. 2B is a sectional view similar to FIG. 2A of an alternativeembodiment of a bidirectional lift gas feed tube having one segment forthe upward flow lift gas and another segment tube for the downward flowof lift gas;

FIG. 3 is a schematic view partly in section of a ground based heliumpressure ballast tank with double slip ring, bidirectional pump, andhelium storage bottles;

FIGS. 4A and 4B are schematic side elevational sectional views of howthe double slip ring adjusts to the change of the orientation of theaerostat or tether while maintaining a tight seal to prevent helium orlift gas leakage through the slip ring;

FIG. 5 is a schematic view partly in section illustrating an alternativeembodiment of a ground based helium ballast tank with a helium scrubber;

FIGS. 6A and 6B schematically illustrate an exemplary process forregulating the helium pressure and lift force in response to diurnalchanges of ambient conditions;

FIG. 7 schematically illustrates an exemplary best mode system withonboard temperature, pressure, and force sensors, fiber optic link, andground control center; and

FIG. 8 is a flow chart of an exemplary procedure for operation of thehelium replenishment and pressure regulation system of a novel aerostat.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS INCLUDING BEST MODE

FIGS. 1 to 4A and 4B illustrate a ground-based helium ballast system fora tethered aerostat 10 formed in accordance with one embodiment of thepresent invention. Referring to FIG. 1, the ground based ballast system100 generally comprises a ground based helium ballast tank having anattendant helium storage tank 101 with optional helium storage bottles109 disposed on the ground 20 including a feed-tube 105 which is eitherembedded within the tether or runs parallel to the tether 102. A doubleslip ring 107 allows the helium feed-tube 105 as well as the rest of thetether 102 to adapt to continuous orientation (yaw) and rotationalchanges of the aerostat and catenary configuration of the tether withoutlosing its helium-tightness. The helium ballast tank 101 and the heliumstorage bottles 109 may be housed in a launch and retrieval vehicle, notshown, on the ground 20.

The tethered aerostat 10 is of a suitable aerodynamic shape, as shown inFIG. 1, in order to minimize wind resistance which can cause the tensionof the tether to increase beyond the level needed to balance the liftforce of the aerostat. The wind resistance force is directlyproportional to the drag coefficient as well as being a stronglyincreasing function of the wind speed relative to the aerostat. Thesmaller the drag coefficient, the lower the wind resistance forcebecomes. The drag coefficient not only depends on the shape of theaerostat, but also on the stiffness of the envelope. Insufficientstiffness would result in the fluttering or wavy deformation of the skinunder strong wind which would drastically increase the drag forcethrough a complicated interaction with the boundary layer turbulence.

The skin stiffness is preferentially accomplished through thepressurization of the helium gas, a process which causes the skin totension through slight stretching. Thus a minimum threshold forpressurization, or super-pressure, must be maintained at all timeswhether the aerostat is aloft, or moored. On the other handpressurization greater than the pressure required to maintain the shapeof the airship results in fatigue, stretching of the envelope andleakage of lift gas.

During daylight hours, the high solar gain causes the helium lift gas toheat up, hence an increase in the helium pressure. If the volume of thehelium gas is kept the same, the increase in helium pressure, whichcould go as high as 70% over its nominal level, could potentiallyovercome the tensile stress limit of the envelope, with the consequentlost of structural integrity of the skin.

At night, the drop in ambient temperature cools the helium gas, reducingits pressure in the process. If the helium gas pressure drops below theambient atmospheric pressure, the skin can no longer maintain itsrigidity, with consequent drastic increase in wind drag which couldpotentially rupture the skin, or stress the tether to the point ofbreakage.

The conventional method of regulating pressure is to open valvesconnected to pressure ballonets to enable the air that resides withinthe ballonet to vent out into the ambient atmosphere at altitude in acontrolled manner during daylight hours to allow the helium gas toexpand, hence the pressure increase in controlled. At night times, thevalve is again opened, and the ambient air is forced into the ballonetwith a blower, pressurizing and expanding the ballonet, which in turnscompresses the helium bag, thereby restoring the pressurization of thehelium gas. Blowers and valve control mechanism are heavy, requiringenergy to operate, and are prone to wear out or malfunction, inconsequence frequent and regular maintenance is mandatory to preventcatastrophic failure of the aerostat while in operation.

The invention in contrast to the conventional prior art employs a groundbased helium ballast system 100 to regulate the helium pressure bycontrolling the pressurization of the ballast tank 101. Referring now toFIGS. 1, 6A and 6B when the pressure difference between the proximal endand the distal end of the feed-tube 105 exceeds the natural pressuregradient under the influence of earth and gravity, the helium gas withinis compelled to flow from the higher pressure end to the lower pressureend. For low to medium altitude tethered aerostat, the gravity inducedhelium pressure differential is considerably less than one tenth of theatmospheric pressure at the ground level; hence the effect of gravity onthe helium gas is inconsequential for the present discussion.

During the daylight heating the lift gas inside the aerostat 10 expandsresulting in the opening of a valve connecting the aerostat 10 to theground based lift gas ballast tank 101. As illustrated in FIG. 6A liftgas is removed from aerostat 10 through the feed tube 105 into lift gasballast tank 101 as represented by arrow 602 where it may be removedpurified or treated as will be discussed later in greater detail.Optional ballonets 606 also may be inflated to assist in the removal oflift gas and in maintaining altitude.

At night the reverse process takes place since cooling of the aerostat10 results in a decrease in pressure which is compensated for by pumpingnew or purified lift gas back to the aerostat 10 through the feed tube105 as illustrated by arrow 604 in FIG. 6B. To assist in adding new orreplenished lift gas and maintaining altitude optional ballonets 606 maybe deflated.

In conventional aerostats a typical leakage rate of the helium gas atambient pressure is about 0.02 to 0.035 cubic feet per square foot ofsurface area per day, which can translate into about 200 cubic feet ofhelium loss per day for an approximately 7,000 cubic feet aerostat.Pressurization of the helium gas raises the leakage rate in proportionto the pressure of the helium gas. Additional leakage of the helium gasthrough the seams and valves increases the total leakage rate slightly,but a well constructed aerostat will keep the leakage rate below 300cubic feet per day. In addition to helium gas leakage, there is also airinfiltration through the skin to contaminate the lift gas and lower thelift force of the lift gas. Since air molecules are far larger thanhelium molecules, the air infiltration rate is typically an order ofmagnitude less than the leakage rate of the helium gas. Still, theperiodic scrubbing of the helium gas to remove the infiltrated air ismandatory ground maintenance in conventional aerostats to ensure thatthe lift force will stay above an acceptable performance level.

In accordance with the invention scrubbing of the lift gas is achievedwhile the aerostat is deployed through a novel tether having a feed tubeof about a half inch in diameter to one and a half inch in diameter. Afeed-tube of approximately half an inch in diameter can move 300 cubicfeet of helium a day with a flow rate of only 1.2 meter per second(m/s). At this flow speed, the total flow resistance contributes to apressure head of only seven percent of the atmospheric pressure,assuming the length of 5000 feet for the tether.

Thus helium maintenance and replenishment can be easily accomplishedwith a relatively small diameter feed-tube of about 1.2 to 4.0 cm (0.5to 1.5 inch) in diameter with a wall thickness of about 0.1 to 0.5 mmprovided that the replenishment operation is performed continuously. If,for example, the helium leak is allowed to accumulate for 20 days beforerefilling or maintenance is performed, then the resistive pressure headcould go as high as 2.7 atmospheric pressure even where the refillingtakes a whole day. The best mode of the invention contemplates acontinuous bidirectional transfer of lift gas assisted by the diurnalheat changes with or without supplemental pumping utilizingbidirectional pump 307 (FIG. 3).

A shorter refilling time would bring the resistance pressure headdrastically higher. This arises from the fact that the flow resistancefor a laminar flow in a tube is linearly proportional to the flow speed,and for turbulent flow the resistance is proportional to ¾^(th) power ofthe flow speed. The fluid flow at low speeds is dominated by laminarflow, whereas at higher speeds the flow is mostly turbulent. Either way,the flow resistance is a rapidly increasing function of the flow speed.Since the higher the flow resistance, the higher the pressure headrequired to move the helium, it is therefore prudent to keep the flowvelocity to a minimum while continuously moving lift gas between theaerostat 10 and the lift gas ballast tank 101 while taking advantage ofthe diurnal temperature differences. Intermittent replenishment of thehelium gas dramatically increases the needed flow velocity; hencecontinuous operation is preferred over intermittent operation.

In order to compensate for diurnal helium pressure swings, the amount ofhelium volume that needs to be moved in either direction (from proximalto distal and vice, versa) is drastically larger. For a tetheredaerostat 70,000 cubic feet in volume operating at an altitude of 5,000feet, about as much as 20% of the helium volume, or about 14,000 cubicfeet, must be cycled everyday. Proper choice of skin material and/orcoating to control the emissivity/absorptivity characteristics of thehull can reduce the diurnal volume swing to less than 14% of the totalvolume, or about 10,000 cubic feet. This would increase the flow speedto more than 50 m/s, which would dramatically ratchet up the resistancepressure head to a level that would make the operation infeasible usinga feed-tube of approximately one half inch in diameter.

This problem can be solved by increasing the diameter of the feed-tubewhich has the effects of simultaneously reducing the flow velocity andthe drag coefficient of the feed-tube. For a given flow speed and agiven material, the flow drag is a strongly decreasing function of thediameter of the feed-tube. For laminar flow, the flow resistance isinversely proportional to the square of the diameter, whereas forturbulent flow, the drag is inversely proportional to the 5/4^(th) powerof the diameter. When the reduction of flow speed with increasingdiameter is taken into account, the drag reduction factor for a givenflow rate (cubic feet per second) can be shown to be inverselyproportional to the 4th power of the diameter for laminar flow and11/4^(th) power for turbulent flow.

Thus a doubling of the diameter of the feed-tube will decrease the flowresistance pressure head by 6.7 to 16 times. For example, if apolyurethane feed-tube is taken to have a diameter of one inch, then amaximum flow velocity of 10 m/s would be capable of moving 7,000 cubicfeet of helium through the feed-tube in about 12 hours. At this speed,the flow is primarily of turbulent nature, and the resistance pressurehead is 48% of the atmospheric pressure at sea level. To overcome such apressure head requires a theoretical pump power of 384 Watts or about4.6 kilowatt-hours in a 12 hour period. The actual pumping powerrequirement is about at least three times higher but still quite easilymanageable. The total weight of the feed-tube composed of polyurethanewould be no more than 200 kg or about 130 grams per meter.

For a one inch (inner diameter) feed tube, a maximum flow velocity of 10m/s can displace 7700 cubic feet of helium every twelve hour, which issufficient for regulating the helium pressure against diurnal thermalcycling in a 70,000 cubic feet aerostat tethered at 5000 feet altitude.The pipe Reynolds number is 2341, exceeding the critical Reynolds numberof 2320 for pipe flow, which makes the flow of helium inside the feedtube a marginally turbulent flow. The total resistance pressure head forsuch a feed tube 5000 feet in length is 24600 Pascal, or about 24% ofthe standard atmospheric pressure, using a typical skin roughness of 1mils.

A polyurethane feed tube provides more strength but at a disadvantage ofweight for example 130 grams per meter. Where the feed tube is made ofeither high density polyethylene with a mass density of 0.95 gram percubic centimeter, or polypropylene having a mass density of 0.91 gramper cubic centimeter weight can be reduced. Both can have a yieldtensile strength as high as 43 mega Pascal. Assuming a safety factor of10, the required feed tube wall thickness is 0.29 mm, or about 11 mils.A 5000 feet long feed tube would then weigh about 32 kg, or 70 lbs. Thiscompares with roughly 1000 lbs weight of the power tether and a weightof about 0.014 pounds per foot of feed tube or about 20 grams per meter.

The prior art does not utilize a bidirectional flow or lift gas butinstead employs a conventional unregulated burst feeding technique usedin the low altitude tethered aerostat which results in much higherresistance pressure head for the feed tube at higher altitudes.Compressed helium usually comes in a steel cylinder which can withstanda pressure differential of 150 psi, or about 10 atmospheric pressure.Since the envelope of the aerostat cannot withstand such high pressure,a special balloon-filler valve/nozzle is often used to reduce thepressure. Still, it can be expected that the pressure of the heliumemerged from such nozzle can be more than 5 atmospheres at its maximum.For the same 1 inch diameter feed tube, such pressure can push thehelium gas inside the tube to slightly more than 50 m/s. At such flowrate, the 300 cubic feet daily helium loss can be replenished in a mere6 minutes. However, the drastically larger pressure head means that thewall thickness of the feed tube has to be at least 6 mm, or 0.236inches. At 5,000 feet, the total weight of the feed tube would be morethan 660 kg, or 1,460 lbs, or about 433 grams per meter for the feedtube alone far exceeding the total weight budget typically allocated forthe power tether at such altitudes or 433 grams per meter.

In contrast, if the helium maintenance operations were to be carried outintermittently, which is the way blower works, with a duty cycle of forexample 0.1, then the flow velocity would be ten times higher, and theresistance pressure head would increase to 27 times the atmosphericpressure. To withstand such a pressure differential, the wall of thefeed-tube would have to be thick and stout and weigh more than 3 metrictons, making it far too heavy and inflexible to be practical.

Referring now to FIG. 2A the feed-tube 105 is disposed coaxially withthe tether 102 and preferably embedded within the tether assembly. Thepreferred embodiment of an embedded coaxial feed-tube construction isdescribed in more detail with reference to FIG. 2A. The tether 102generally comprises a jacket 202, a multitude of high tensile strengthinorganic, organic or synthetic fibers 204 such as KEVLAR® or VETRAN®distributed throughout the interior of the tether, a plurality of highvoltage insulated electrical cables 206 for the delivery of electricpower to the aerostat, and one or more telecom grade optical fibers 208for high-speed communications between the aerostat and the ground 20.Inorganic fibers such as glass or carbon may be employed or syntheticfibers such as KEVLAR® a proprietary trademark for aromatic polyamidefibers available from E.I. DuPont deNemours and Company and VETRAN® aproprietary trademark for fibers made of liquid crystal polymersavailable from Hoechst Celanese Corporation. A launch and retrievevehicle may also be employed to serve as the ground station for moremobile operations. Only the high-strength fibers 204 should be undertension, all other members including the feed-tube 105 should follow aslightly helical or catenary trajectory along the longitudinal directionof the tether to ensure that they are not tensioned.

The bidirectional flow of lift gas can be accomplished utilizing asegmented or two separate feed tubes as illustrated in FIG. 2B. Thisalternative embodiment of the invention has advantages in providingseparate upward flow path 105A and a downward flow path 105B but has thedisadvantages of increasing weight and cost of fabrication. Instead ofadding weight by utilizing a segmented or a plurality of feed tubes thebest mode of the invention contemplates the use of a bidirectional pumpin the ground based ballast cavity 302 as will be discussed hereinafterin greater detail.

The helium ballast tank 101, as illustrated in FIG. 3, is a ground basedballast chamber or large pressurized cavity 302 attached to thefeed-tube 105 through a double slip ring connector 107, and is connectedto a second smaller pressurized cavity 305 through a bi-directionalpositive displacement pump 307. The second cavity 305 is in turnconnected to a plurality of high pressure helium storage tanks 109through computer controlled valves 311.

The helium pressure within the ground based ballast cavity 302 isautonomously controlled by the bi-directional positive displacement pump307 which pumps the helium from the second smaller pressurized cavity305 to the ground based ballast cavity to increase the pressure of thelatter and pumps the helium from the main ground based ballast cavity302 back to the second smaller pressurized cavity 305 when a reductionof the helium pressure of the ground based ballast cavity 302 isrequired. To minimize the stress and power consumption of the pump 307,the helium pressure inside the second smaller pressurized cavity 305 ismaintained to be approximately the same as that in the main ground basedballast cavity 302 by controlling the opening and closing of the valves311 to admit just enough helium gas to replenish the net helium lossfrom the aerostat 10. The control of the valves 311 is intermittent inorder to reduce the wear and tear of the valves.

A hysteresis is built into the control mechanism to prevent theincessant rapid opening and closing of the valves with a singlethreshold-based control mechanism. Since the helium gas leakage rate isless than 300 cubic feet a day, the valves 311 need only be openedbriefly once every few days. In contrast, the pump 307 needs to beoperated continuously to compensate for the diurnal changes in heliumpressure of the aerostat 10. This causes the pressures inside the mainground based ballast cavity 302 and the second smaller pressurizedcavity 305 to fluctuate by as much as 30% daily.

A free-rotating structure such as the aerostat 10 has to be connected toa stationary structure such as the ground or a launch and retrievevehicle through one or more slip rings. Mechanically, the slip ringreleases the stress buildup when the aerostat rotates by a multiple morethan 360 degrees relative to the ground structure. The slip ring alsoenables the electric cables inside the tether assembly to maintainelectrical contact with the ground power supply. In accordance with theinvention the helium feed-tube 105 also must be connected to the groundhelium ballast 101 in a gas tight fashion to minimize helium gas leak.This object is impossible to achieve with the prior art slip ringarrangement for airships since such an arrangement invariably requires asliding gas seal.

A sliding seal has to satisfy two conflicting objects; i.e. the high gassealing capability and the ease of rotation. To ensure high gas sealingperformance, the seal must be pressed against the rotating structurewith a large force to keep the gap between the rotary sleeve and thestationary sleeve to a minimum, but this would unavoidably increase thefriction between the rotary sleeve and the stationary sleeve to thepoint of making any relative rotational movement difficult. Anylessening of the contact pressure between the rotary element and thestationary element would drastically increase the helium leakage.

This problem has been solved in accordance with the invention byemploying a double slip ring as illustrated in FIGS. 4A and 4B. Thedouble slip ring 107 includes a first slip ring 402 and a second slipring 405, interconnected by a flexible connecting tube 407 in the shapeof a flexible coil 408 with a plurality of turns. Both slip rings 402and 405 are made of a ferromagnetic alloy which exhibits a strongpermanent magnetic field, and are housed inside a low pressure chamber415. Normally, the first slip ring 402 is allowed to freely rotate FIG.4B with a portion of feed tube 105 while turning is absorbed by coil 408and while maintaining a modest gas sealing performance while the secondslip ring 405 is kept stationary by a strong electromagnetic fieldgenerated by a first electromagnetic coil 417. The high contact pressureproduced by the electromagnetic force ensures that the second slip ring405 can provide a tight seal for the helium. The flexible coil structureof the connecting tube 407 allows the first slip ring 402 to rotate bymultiple 360 degree turns without rotating the second slip ring 405.

When a preset maximum number of turns is exceeded, the first slip ring402 is momentarily kept stationary with a second electromagnet 419 toprovide a tight helium seal FIG. 4B, and the second slip ring 405 isreleased from the electromagnetic force generated by the firstelectromagnet 417 and is rotated by exactly the same number of turnsquickly to remove the tension built up on the connecting tube 407 andreturn flexible coil 408 to its previous configuration. Any heliumleakage from the second slip ring 405 during the brief rotation isrecovered through a suction pump (not shown) that is connected to theopening 413 to the low pressure chamber 415. Since the first slip ring402 inevitably admits ambient air to infiltrate, the low pressurechamber 415 contains a mixture of helium and air, hence a scrubber isused to remove the infiltrated air before the helium is fed back intothe second smaller pressurized cavity 305.

The helium pressure regulation for diurnal temperature compensationrequires the movement of helium in both directions. During the daytimethe aerostat helium temperature increases owing to solar heat gain, andthe ground based helium ballast tank 101 must lower its helium pressureto allow helium gas to move from the aerostat 10 to the largepressurized cavity 302. A portion of the infiltrated air in the aerostatwill also be moved from the aerostat 10 to the ground based regulatedballast cavity 302.

This makes it possible to do maintenance on the helium lift gas andscrub the infiltrated air from the helium gas on the ground without theneed to take the aerostat down. Assume, by way of example, a typical airinfiltration rate of 50 cubic feet per day, given that 1/7^(th) of thehelium volume in the aerostat 10 is cycled everyday, the entire heliumvolume will be replaced every 7 days. It thus follows that with acontinuous scrubbing operation, a steady state is achieved when theinfiltrated air is approximately 350 cubic feet in volume. This can bededuced from the fact that when the air is less than 350 cubic feet involume, the air removed through scrubbing per day will be less than 50cubic feet; hence the air volume will continue to grow. Likewise, whenthe infiltrated air is more than 350 cubic feet in volume, the airremoved daily by scrubbing will be more than 50 cubic feet per day,which exceeds the air infiltration rate, and thus the air volume willdecrease.

FIG. 5 illustrates the maintenance of the helium lift gas on the groundwhile the airship is deployed at altitude wherein the inlet 504 of thehelium scrubber 501 is connected through a compressor to the smallerpressurized cavity 305 and its outlet 506 is also connected to thesmaller pressurized cavity 305. The compressor 503 continuously drawshelium-air mixture from the smaller pressurized cavity 305 and sends thecompressed mixture through the semi-permeable membrane 509 inside thescrubber chamber 510 to separate the air from the helium. The separatedair is released to the atmosphere, and the pure helium gas is returnedto the smaller pressurized cavity 305. The compression also increasesthe dew point inside the chamber 510, wherein the moisture contained inthe mixture is condensed and is bled to water drainage 521 through ableed valve 522 periodically.

The dehumidified helium gas can help prevent frosting when it isreturned to the airship deployed at altitude since frosting is adangerous condition when the temperature of the aerostat skin dropsbelow freezing point. Frosting on the skin can make the portion of theskin coated with a thin layer of ice brittle by creating high stresspoints under external load. Frosting only occurs when the inside of theairship skin is coated with condensate before the skin temperature dropsbelow freezing point. With the lower moisture content, condensationcannot take place inside the aerostat. The outside skin can stillcollect condensate if the ambient temperature drops below dew point.This is unlikely to happen since a prevalent wind at altitude preventscondensate from accumulating on the skin. On the rare occasion of awindless night, the air is usually dry since the wind generally receivesits energy from the latent heat contained in moisture. It would also bepossible to preheat the helium gas inside the ballast chamber 302 atnight to maintain a slightly elevated skin temperature above the ambienttemperature to prevent condensation.

The regulation of helium pressure against diurnal temperature swing hasthe benefit of minimizing the cycling of skin surface tension which candrastically reduce the service life of the skin. This is because a largefluctuation of skin tension can accelerate the fatiguing of the skinmaterial through creep, a gradual irreversible deformation of theenvelope that results from repeated application of mechanical stress.With advanced feedback control, it is possible to maintain nearly aconstant helium pressure irrespective of the ambient conditions.Although this would minimize the cycling of the pressure induced stress,it would also lead to a diurnal swing of the total lift force on theaerostat 10. This is owing to the fact that when the helium temperatureis low, particularly at night, more helium is needed in order to achievethe same gas pressure. This would make the aerostat heavier because ofthe added helium mass, as illustrated in FIG. 6B. The small molecularweight of the helium lessens the diurnal swing of the lift force to lessthan 3% of the total lift force. A further reduction of the lift forcefluctuation can be realized if a second, smaller feed-tube connects eachof the ballonets 606 and is used to regulate the air pressure inside theballonets 606. By removing the same mass of air from the ballonets asthe mass of the helium added to the aerostat, the total mass of theaerostat stays constant; hence the total lift can remain unchangedthrough diurnal variations.

Referring now to FIGS. 3 and 7 to efficiently regulate helium pressurein the aerostat, a computer 701 on the ground is used to control thepump 307 (FIG. 3) in ballast tank 101. Valves 311 and pressure sensor314 (FIG. 3) are utilized together with pressure and temperature sensor704 to monitor the helium pressure and temperature inside the aerostat10 as well as the skin and ambient air temperatures and the temperatureand pressures inside the ballast chamber 302 and smaller pressurizedcavity 305 as inputs to a control program in order to control the speedand direction of operation of the helium bidirectional positivedisplacement pump 307 and the time and duration of the opening of thevalves 311 in order to provide optimal regulation of the helium and gaspressures and also possibly the heating of the lift gas in aerostat 10to control the lift force.

The pressure and temperature monitoring is provided by an array ofonboard pressure and temperature sensors 704. These sensors 704constantly relay environmental data to the computer 701 through one ofthe optical fibers 208 (FIG. 2) embedded inside the tether 102, as shownin FIG. 2. Similar sensors (not shown) are embedded inside the ballastchamber 302 and smaller pressurized cavity 305, and inside the heliumscrubber 501 and the helium storage bottles 103. The data collected bythose earth bound sensors are sent to the control computer 701 through alocal area network (not shown).

After the sensor data is processed by the computer program, the computer701 sends control parameters to a digital controller 711 which convertsbinary data into control voltages and/or currents to drive the heliumbidirectional positive displacement pump 307, open and close valves 311,and the compressor 503 that provides compressed helium gas to thescrubber 501. The control program could be PDI(principle-differential-integral) based, or could be model basedpredictive/feedback control program, or any other mathematical feedbackcontrol algorithm that can provide stable, accurate control. Of these,the model-based control is believed to be the best mode because it isbased on a mathematical model of the physical processes that govern thepressure variations of the individual subsystems such as the aerostat 10and the ballast system 100 (FIG. 1).

A flow chart of the ballasting program is shown in FIG. 8. FIG. 8 alsoillustrates the best mode of the invention in which pressure andtemperature sensors 704 (FIG. 7) provide data as represented by block802 to a computer 701 to compute helium buoyancy force and total lift asrepresented by block 804. An optional temperature and pressure sensor isprovided for ballonets 606 which also provides data as represented byblock 806 to computer 701 which is used in calculating buoyancy forceand total lift as illustrated in block 804 to take into accountdifferences in diurnal variations.

Once buoyancy requirements are calculated the actual helium pressure iscompared with the calculated pressure as represented by block 808. Ifthe calculated helium pressure is greater than the actual set pressure,pressure in ballast tank 101 is increased until the pressure in theaerostat is within the normal range as indicated by block 810. On theother hand if the actual helium pressure is greater than the calculatedpressure, pressure in ballast tank 101 is reduced to the normal range asrepresented by block 812.

In the event the total lift is less than the lift minimum as representedby block 814, the aerostat is brought down for maintenance asrepresented by block 816.

The method of the invention and flow chart can include sensors in thehelium ballast tank 101 and in the helium bottles 109 (FIG. 9) to sampleand compare the purity of the helium lift gas as well as the humidity ofthe lift gas in making calculations as to lift, temperature and pressurein the aerostat 10 and well as in the helium ballast tank in monitoringthe efficiency of the helium scrubbers as well as providing informationas to the quality of the lift gas received from the aerostat as well asthe quality of the lift gas supplied to the aerostat.

The novel aerostat as supported by the novel tether and slip rings canbe operational for extended periods of time without having to be broughtdown for service thanks to the continuous replenishment, purification,and dehumidification of the helium gas and the minimal use of theballonet valves and the absence of prior art blowers. However, it mayeventually be necessary to retrieve the aerostat for repair or becauseof bad weather, or to update the sensors or communication payloads. Inorder to move a substantial percentage of the helium gas from theaerostat to the ballast tank within a reasonable time of one or twohours that it requires to recover the aerostat would mandate a furtherincrease of the diameter of the feed-tube. Similar consideration appliesto the launch of the aerostat.

It will be recognized the invention is capable of numerous changes andmodifications by those skilled in the art. The airship may be equippedwith lightweight engines to assist in launch recovery or deployment andthe lift gas may be modified to employ mixtures of lift gasses and thearrangement of the components in the tether may be modified as well asthe organization of the components of the slip rings to accommodate therotational contact of the optical fibers and electrical cables as wellas the way in which control signals are sent to and processed on theground and in the aerostat. These and such other variations are intendedto be included in the scope of the appended claims.

As used herein and in the following claims, the word ‘comprising’ or‘comprises’ is used in its technical sense to mean the enumeratedelements include but do not exclude additional elements which may or maynot be specifically included in the dependent claims. It will beunderstood such additions, whether or not included in the dependentclaims, are modifications that both can be made within the scope of theinvention. It will be appreciated by those skilled in the art that awide range of changes and modification can be made to the inventionwithout departing from the spirit and scope of the invention as definedin the following claims:

1-18. (canceled)
 18. A slip ring for accommodating a weathervaningaerostat comprising: (a) a housing having a first end and a second end;(b) a first rotatable element disposed in said first end of saidhousing; (c) a second rotatable element disposed in said second end ofsaid housing; (d) a flexible coiled tubing connecting said firstrotatable element to said second rotatable element; (e) anelectromagnetic coil to maintain said second rotatable elementstationary with respect to said first rotatable element until apredetermined number of revolutions of said first rotatable element isexceeded.
 19. The slip ring of claim 18 further comprising a secondelectromagnetic coil to reduce the rotatability of said first rotatableelement.
 20. The slip ring of claim 19 wherein said housing issubstantially airtight.
 21. The slip ring of claim 20 wherein thehousing includes a port for recovering a gas leaked inside said housing.22. The slip ring of claim 20 wherein said first rotatable plateincludes a tapered perimeter.
 23. The slip ring of claim 18 wherein saidsecond rotatable element is of a frustro conical configuration.
 24. Theslip ring of claim 23 wherein said housing includes a mating frustroconical opening for said second rotatable element.
 25. The slip ring ofclaim 24 wherein said electromagnetic coil is disposed around theoutside of said mating frustro conical opening.
 26. The slip ring ofclaim 18 further comprising a tether with a hollow feed tube and atethered aerostat with a ground based lift gas ballast tank wherein saidslip ring is disposed intermediate said tethered aerostat and saidground based lift gas ballast tank.
 27. An airship tether comprising:(a) a hollow coaxial lift gas feed tube providing a bidirectional flowof a lift gas and having a wall thickness of 4 about 0.1 to 0.40centimeters; (b) a fibrous strengthening member surrounding said 6hollow coaxial lift gas feed tube; (c) an electrical cable disposed insaid fibrous strengthening member; and (d) at least one optical fiberdisposed in said strengthening member.
 28. The airship tether of claim27 wherein said hollow coaxial feed tube has a diameter of about 1.2 to4.0 centimeters.
 29. The airship tether of claim 27 wherein said hollowcoaxial feed tube weight is about 15 to 140 grams per meter.
 30. Theairship tether of claim 27 wherein said strengthening member is selectedfrom a group of strengthening members consisting of Kevlar, Vectran,glass or carbon fibers.
 31. The airship tether of claim 27 wherein saidelectrical cable is a plurality of electrical cables.
 32. The airshiptether of claim 27 wherein said optical fiber is a plurality of opticalfibers.
 33. The airship tether of claim 27 wherein said hollow coaxialfeed tube is axially divided into two coaxial feed tubes to provide saidbidirectional flow.
 34. A method of extending the duration of deploymentof a tethered aerostat comprising: (a) employing a ground based lift gasreservoir; (b) utilizing a hollow lift gas feed tube in a tetherconnected to said ground based lift gas reservoir; (c) deploying atethered aerostat connected to said hollow lift gas feed tube; and (d)providing a bidirectional flow of lift gas between said tetheredaerostat and said ground based lift gas reservoir.
 35. The method ofclaim 34 further comprising the step of utilizing diurnal pressure andtemperature variations to assist said step of having a bidirectionalflow of lift gas.
 36. The method of claim 34 further comprising the stepof purifying a lift gas in said ground based lift gas reservoir.
 37. Themethod of claim 34 further comprising the step of replenishing a liftgas in said ground based lift gas reservoir.
 38. The method of claim 34further comprising the step of employing a slip ring between said hollowlift gas feed tube and said tethered aerostat.
 39. The method of claim38 wherein said slip ring is substantially airtight.
 40. The method ofclaim 39 wherein said slip ring is connected to said ground based liftgas reservoir.
 41. The method of claim 34 further comprising the step ofcollecting environmental data from said tethered aerostat.
 42. Themethod of claim 34 further comprising the step of collecting pressureand temperature data from said tethered aerostat.
 43. The method ofclaim 34 further comprising the step of computing lift gas pressure andlift force.
 44. The method of claim 43 further comprising the step ofcomparing lift gas pressure with computed lift gas pressure and liftforce.
 45. The method of claim 44 further comprising the step ofincreasing or decreasing lift gas pressure in said ground based lift gasreservoir to change lift gas efficiency in said tethered aerostat. 46.The method of claim 44 wherein said lift gas is helium.
 47. The methodof claim 34 further comprising the step of scrubbing said lift gas insaid ground based lift gas reservoir.
 48. The method of claim 34 furthercomprising the step of dehumidifying said lift gas in said ground basedlift gas reservoir.
 49. The method of claim 34 further comprising thestep of utilizing a bidirectional pump to assist in said bidirectionalflow of lift gas between said tethered aerostat and said ground basedlift gas reservoir.
 50. The method of claim 34 further comprising thestep of utilizing a computer to compare the quality of the lift gas insaid ground based lift gas reservoir and the lift gas pressure and forcein said tethered aerostat as measured by sensors in said tetheredaerostat.
 51. The method of claim 34 further comprising the step ofhaving electrical cables and optical communications cables embedded in atether surrounding said hollow lift gas feed tube.
 52. The method ofclaim 51 wherein said lift gas is helium. 53.-87. (canceled)
 88. Adouble slip ring for accommodating a weathervaning airship comprising:(a) a housing having a first end and a second end; (b) a first rotatablegas self sealing slip ring disposed in said first end of said housing;(c) a first sliding seal disposed between said first rotatable gas selfsealing slip ring and said first end of said housing; (d) a secondrotatable gas self sealing slip ring disposed in said second end of saidhousing; and (e) a second sliding seal disposed between said secondrotatable gas self sealing slip ring and said second end of saidhousing.